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Alumni ProjectDevelopment of an Atmospheric Climate Model with Self-Adapting Grid and Physics J. E. Penner, SummaryOne of the most important advances needed in global climate models is the development of models that can reliably treat small-scale convective processes embedded in a large-scale flow. This project will result in a climate model that self-adjusts the horizontal grid resolution and the complexity of the physics module to the atmospheric flow conditions. To accomplish this, we expect to couple a hydrostatic and a non-hydrostatic dynamical core that utilize adaptive grid techniques with block-structured data layouts.1. Introduction The goal of this research project is to develop adaptive grid techniques for future climate model and weather predictions. This approach will lead to new insights into small-scale and large-scale flow interactions that are unresolved by current uniform-grid simulations. Adaptive mesh refinement (AMR) techniques provide an attractive framework for atmospheric motions since they allow improved horizontal resolution in a limited region without requiring a fine grid resolution throughout the entire model domain. Therefore, the model domain to be resolved with higher resolution is kept at a minimum, greatly reducing computer memory and speed requirements. Adaptive grid techniques are applied to a parallel version of NASA's next generation climate model, the NASA / NCAR Finite-Volume Community Climate Model, which has been developed at the NASA Goddard Space Flight Center (GSFC, Data Assimilation Office). This global hydrostatic model is based on NCAR physics and the so-called Lin-Rood finite-volume dynamical core that provides highly efficient algorithms for high performance computing. This research project is characterized by an interdisciplinary approach involving atmospheric science, computer science and mathematical/numerical aspects. The work is done in close collaboration between the Atmospheric Science, Computer Science and Aerospace Engineering Departments at the University of Michigan and NASA. 2. The Adaptive Grid Library The newly developed version of the NASA finite-volume dynamical core with self-adaptive grid utilizes a parallel program library for block-wise adaptive grids on the sphere. As indicated in figure 1, the regular longitude-latitude grid is subdivided horizontally into self-similar blocks that contain an identical number of grid points per block. In the event of a refinement
Figure 1 : Adapted grid with block-data structure. request a block is split into four new blocks thereby doubling the spatial resolution. Here the spatial resolution of adjacent blocks is only allowed to differ by a factor of two. In addition, an initial reduced grid setup can be selected. Then the longitudinal resolution in polar regions is coarsened which alleviates the convergence of the meridians at the poles. The adaptive spherical grid library also provides communication routines for ghost cell updates on parallel computer architectures. Necessary interpolations among neighboring blocks at different refinement levels are provided by user-defined interface routines. 3. The Hydrostatic Dynamical Core Statically and dynamically adaptive grids have been successfully implemented and tested in 2D shallow water simulations and 3D hydrostatic dynamical core runs on the sphere. Figure 2 shows an example of a 2D shallow water simulation at model day 10.
Figure 2 : Geopotential height field at day 10. The depicted geopotential height field is characterized by a lee-side wave that is induced by an idealized mountain. Here a combination of statically and dynamically refined blocks is presented. The dynamic adaptations track the evolution of the wave by a gradient-based adaptation criterion. Other adaptation criteria that are, for example, based on vorticity have also been successfully applied. Tests with the adaptive 3D dynamical core suggest that adaptations are a viable option for future modeling studies. 3D idealized experiments with locally refined resolutions along storm tracks have already been performed. The results show that the developing storm systems are predicted accurately without the need for a fine resolution in the entire model domain. 4. The Non-Hydrostatic Dynamical Core The explicit treatment of convection requires spatial resolutions at which the hydrostatic assumption is no longer valid. Therefore we will couple a non-hydrostatic code to the model that will replace the hydrostatic treatment if required. The non-hydrostatic code is based on an extension of the hydrostatic code. A mass based Lagrangian vertical coordinate replaces the pressure based Lagrangian vertical coordinate of the hydrostatic code. In order to predict the non-hydrostatic pressure the specific volume is added as a prognostic quantity. For non-hydrostatic conditions we developed a modified advection scheme and generalized the calculation of the pressure gradient force. The development of the non-hydrostatic code is in its final stage. The procedure to couple hydrostatic and non-hydrostatic regions is ready for testing.For further information contact:
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